An a.c. generator for a vehicle is in general driven by an engine through a V belt and the like to convert rotational energy into electrical energy to thereby output the electrical energy thus obtained. In recent years, a generator having a large output has been required as an electrical load on a vehicle increases. However, if the output of the generator increases, then the load torque of the engine which is developed by driving the generator increases all the time. For this reason, in particular, for a period of time in which the explosion and combustion are unstable as right after the engine having been started, the revolution of the engine is liable to be in an unstable state. In addition, upon starting the engine, the generator outputs a maximum output in order to charge a battery which has been over-discharged by activating a starter motor, so that the load imposed on the engine is increased all the more and hence the revolution of the engine is more and more liable to be led into an unstable state. Such a phenomenon is particularly remarkable in the cold time.
Heretofore, in order to solve the inconveniences as described above, a control device has been proposed in Japanese Patent Application Laid-open No. Hei 3-173324, in which for a predetermined period of time after an a.c. generator has started power generation, the conducting rate of a switch connected in series with a field coil is set to a minimum value, whereby the load torque which is generated by driving the generator just after the starting of the engine is perfectly excluded to achieve the stabilization of engine revolution, and also after a lapse of a predetermined time, the setting of the above-mentioned conducting rate is controlled in such a way as to gradually increase from the above-mentioned minimum value up to a maximum value to prevent a drastic increase in the engine load as well as the generation of belt slipping noise in the cold time, and also to prevent the generation of belt slipping noise and a reduction of engine revolution when the generator output is recovered.
A circuit diagram of such a control device for a vehicular a.c. generator is shown in FIG. 6 and a control characteristic diagram is shown in FIG. 7.
The operation of this control device will hereinafter be described.
Reference numeral 1 designates a generator; reference numeral 2, a rectifier; reference numeral 3, a voltage regulator; reference numeral 4, a field current controller; reference numeral 5, battery; reference numeral 6, a key switch; reference numeral 7, an electrical load applied to a vehicle; reference numeral 8, a switch; and 10, an initial exciting resistor.
Since at a time point when the key switch 6 has been turned on, the generator 1 does not yet start power generation, a comparator 307 of the voltage regulator 3 is in the low state and a timer 402 of the field current controller 4 is also in the low state. Therefore, at this time, a comparator 409 of the field current controller 4 is in the high state and a power transistor 310 is turned on so that an initial exciting current is caused to flow through a field coil 102 via the initial exciting resistor 10. Then, at the time when the generator 1 is driven by the engine to start power generation, the timer 402 becomes the high state to start the measurement of time. For a predetermined period of time T1 in which the timer 402 is in operation, the voltage at a point C is at a level which is set by voltage division resistors 403 and 404.
The relationship between this setting level and a chopping wave signal at a point B is as shown at (a) of FIG. 7, and those signals are compared with each other in a comparator 409 of the field current controller 4, and on the basis of the comparison output, a conducting waveform of the power transistor 310 as shown at (b) of FIG. 7 is obtained. At this time, the conducting rate of the power transistor 310 is at the minimum setting value and the power generation voltage resulting therefrom is made equal to or lower than the voltage of the battery. Therefore, for the predetermined period of time T1, as shown at (c) of FIG. 7, the output of the generator 1 is zero.
Next, at the time when after a lapse of the predetermined period of time T1, the timer 402 becomes the low state, the voltage at point C is multiplied by another predetermined period of time T2 which is set by a time constant circuit comprising a capacitor 407 and a resistor 406 to be decreased down to a predetermined level as shown at (a) of FIG. 7. The conducting waveform of the power transistor 310 which is obtained on the basis of the comparison output of the comparator 409 for this period of time is as shown at (b) of FIG. 7, and hence the conducting rate of the power transistor 310 is gradually increased to reach the maximum value (100%) after a lapse of the predetermined period of time T2. Then, for the predetermined period of time T2, the output of the generator is gradually increased from 0 up to 100% as indicated by a solid line at (c) of FIG. 7. Thereafter, the operation proceeds to the original control based on the comparison output of the comparator 307 of the voltage regulator 3.
In this prior art, the period of time T1, in which the load torque generated by driving the generator right after the engine having been started is removed to stabilize the revolution of the engine, and the period of time T2, following the period of time T1, in which the output of the generator is gradually increased up to 100%, are both made to the fixed values, respectively.
However, such control on the generator is required only at low temperatures and hence it is unnecessary at high temperatures because the startability of the engine is better at hot time than at cold time.
If, however, the generator is controlled at high temperatures, the output of the generator is suppressed right after the engine having been started, so the battery 5 is discharged, thus resulting in a reduction of the charging performance thereof. This is undesirable.
Therefore, it is preferable that at high temperatures, the period of time T1 in which the output of the generator is suppressed and the period of time T2 in which the output of the generator is gradually increased are both shortened to speedily increase the output of the generator up to 100%.
In addition, while for a period of time required to detect the start of the engine (for a period of time in which the revolution of the engine is equal to or lower than a threshold), i.e., during an initial excitation period, the output of the generator is suppressed to the low level to make the load applied to the battery small, it is preferable that the initial excitation period be shortened since the startability of the engine is better at hot time.